Neutron exposure of workers in the nuclear industry, well loggers, and airline crew members, includes significant doses of neutrons with energies below 100 keV. In addition, the normal tissue dose from boron neutron capture therapy will largely be from low-energy neutrons. Microdosimetric theory predicts decreasing relative biological effectiveness (RBE) for neutrons with energies below about 350 keV compared to that for higher energy neutrons; this is because interactions of low-energy neutrons with biological material give rise to recoil protons in tissue that have both decreasing LET (stopping power), and decreasing range. Based on such considerations, and limited biological data, the currently assigned radiation weighting factor (previously the quality factor) for neutrons with energies from 10 keV to 100 keV is less than that for higher-energy neutrons. By contrast, a few reports have suggested that the biological effects of 24 keV neutrons is similar to that of fast neutrons. Both cell survival and oncogenic transformation induction were determined for C3H10T1/2 cells exposed to two low-energy neutron beams with dose-averaged mean energies of 40 and 70 keV, and compared to that for higher energy neutrons and X rays. The results were consistent with the currently recommended decreased values for low-energy neutron radiation weighting factors, compared to fast neutrons.